A Better Wall with Exterior Foam
Why a longtime SIP builder turned to rigid polyiso for insulation.
Synopsis: Builder Jonathan Orpin was a diehard fan of SIPs and used them in countless projects. Over the years, though, he started to realize that manipulating SIPs for sidewalls required a lot of work when cutting openings and routing mechanicals. To get around this problem, Orpin developed what he calls the matrix wall, which consists of conventionally framed 2×6 walls with either wet-sprayed or netted dense-pack cellulose in the cavities and polyisocyanurate foam in the exterior. Housewrap and a rainscreen of 1x strapping serve as a weather-resistant barrier. Orpin says the assembly yields an R-30 wall with negligible thermal bridging and a managed dew point. This article includes sidebars on controlling moisture in walls and a better way to install windows.
I’ve been a home designer, builder, and timber framer for more than 25 years, and I recently calculated that my projects have consumed almost 2 million sq. ft. of SIPs (structural insulated panels). I still use SIPs almost exclusively for roofs, where they serve as insulation, short-span structure, and premade overhangs.
When my firm used SIPs for sidewalls, however, there was a great deal more work involved in cutting openings and routing mechanicals, so almost 20 years ago, we started developing an alternative. Our goal was to find an open-cavity option that made installing mechanicals easier and was still well insulated.
The solution, which I refer to as the matrix wall, consists of conventionally framed 2×6 walls with either wet-sprayed or netted dense-pack cellulose in the cavities and polyisocyanurate foam on the exterior. Instead of using the foam as the weather-resistive barrier, I install housewrap and a rainscreen of 1x strapping. The assembly yields an R-30 wall with negligible thermal bridging and a managed dew point.
Of course, thermal performance means little if the walls allow water and air infiltration, and establishing durable…